This invention relates to packings and more particularly to braided packings comprising a plurality of strands braided together and further relates to a method and apparatus for making braided packings.
Braided packings are used extensively in industry for a variety of sealing purposes. Typically packings are made of flax, jute, asbestos or synthetic, such as polytetraflouroethylene, fibers which are formed into yarns or strands and which are braided together. The result is typically a packing having a square cross-section and herringbone weave pattern extending in an axial direction along the packing; typical such packings are illustrated in U.S. Pat. No. 3,646,846.
When standard prior art packings are wrapped around a shaft, however, they do not provide a uniform fiber density. As they are wrapped about a shaft, the packing fibers at the inner diameter are compressed and those at the outer diameter are elongated. The result, with an initially square cross-section packing, is a trapezoidal cross-section providing a leakage path larger at the outer diameter than at the inner diameter. Additionally, the sealing characteristics of typical packings are static and substantial compression is required to effect sealing. Compression of the packing to control leakage causes increased pressure on the shaft and forces out lubrication from the packing, overheating and shortening the life of the packing and contributing to shaft wear.
In application Ser. No. 754,254, of which this is a continuation-in-part, a braided packing is disclosed which has a normal helical twist. The packing has a density of improved uniformity requiring less pressure for sealing, thereby enhancing packing and shaft life. The packing also functions to provide hydrodynamic sealing characteristics when properly oriented relative to shaft rotation, further reducing sealing pressure required and further contributing to extended packing and shaft life.
The method and apparatus disclosed in application Ser. No. 754,254, however, limit the style of the twisted braid which can be made. Thus, in making the braid, only the principal braid strands, all extending in the same helical direction, could be employed; warp strands extending helically in the opposite direction could not be employed due to inherent limitations in the disclosed method of making the braid which requires disabling the track normally carrying warp strands. The inability to utilize warp strands in the braid limited the size and the density of packings which could be made with a normal helical twist. Additionally, because of the normal direction of twist in the yarns employed as braid strands, it is desirable that operation of braiding machinery be limited to braid the braid strands in one direction; thus suitable braid would be produced having a helical twist only in a single direction.